In production processes, spectrometric measurements can be performed in gases, liquids, solids, and multiphase mixtures in order to obtain knowledge about the production process or about a substance formed as a product of the process, for example, its quantity and quality. From spectrometric measurements, values of measurands correlating to the concentration of educts and/or additives of the process can also be obtained. For example, in a biochemical production process, concentrations of nutrients and/or concentrations of metabolites of the microorganisms used in the production process and/or the concentration of the product produced in the process in a process medium can be monitored, and the process sequence can be controlled and/or regulated based upon the measured data obtained. The process medium is generally contained in a process container, such as a reactor, a fermenter, or in a duct.
A spectrometric method very well-suited for analyzing and monitoring gaseous, solid, and liquid process media is Raman spectroscopy. It is based upon the inelastic scattering, called the Raman effect, of electromagnetic radiation by atoms or molecules. The largest portion of the radiation radiated into a measuring medium is elastically scattered by the molecules of the measuring medium as so-called Rayleigh scattering. This portion of the scattered radiation has the same wavelength as the excitation radiation. In the inelastic scattering by molecules of the sample, an energy transfer takes place, wherein a molecule interacting with the excitation radiation can transition via a virtual state into an energetically higher state (Stokes scattering) or into an energetically lower state (anti-Stokes scattering). In the first case, energy is consumed, so that the scattered radiation has a lower energy than the excitation radiation. In the other case, energy is released, so that the scattered radiation has a higher energy than the excitation radiation. A Raman spectrum is an illustration of the intensity of the inelastically scattered radiation as a function of its frequency difference from the excitation radiation (generally specified in wavenumbers, cm-1). The Raman spectroscopy is a vibrational spectroscopy, i.e., the energy transfers detected using Raman spectroscopy correspond to characteristic vibration energy levels of the molecules or their functional groups. Thus, based upon certain peaks or bands in the Raman spectrum, the presence of certain molecules in the sample and, based upon the intensity of the respective peaks or bands, their concentration, can be determined.
Especially advantageous in Raman spectroscopy in connection with process media containing water, for instance, biological systems or biotechnological processes, is the fact that water is a very weak Raman scatterer, so that Raman signals of molecules dissolved in water can be seen easily in the Raman spectrum of the solution. In addition, Raman spectroscopy does not require any additional preparation of the sample and can provide measured values in a short time. This method is thus especially attractive for process analysis and process control.
In the prior art, it is customary to take samples of a process medium from the process container and to examine them in the laboratory by means of a spectrometer in order to determine values of the respective measurands to be determined from the spectral data obtained. The spectra detected using the spectrometer can be analyzed by means of a data processing unit, e.g., a conventional computer. Problematic in this case is the sampling, since it results in a significant time delay between the taking of the sample from the process container and the availability of the final measured value. Moreover, the taking of samples from processes to be kept sterile, e.g., in food technology and in processes of the pharmaceutical industry and/or of biotechnology, is associated with a high investment in equipment and personnel in order to take the samples properly and without contaminating the process. Depending upon the type of process, a health hazard can also exist during the sampling, if an undesired contact of the sample or the process medium with the environment of the process container occurs.
From U.S. Pat. No. 5,862,273 is known a Raman spectrometer with a probe, which can be integrated as a spectrometric interface as an inline probe into the process container. The probe is connected to additional components of the spectrometer, such as a laser radiation source and a spectrograph via optical waveguides in the form of optical fibers. Disadvantageous in such a spectrometric measuring device with a probe connected to the actual spectrometer, and possibly an additional evaluation unit via optical waveguides, is the fact that such a wave guide connection cannot be realized over any arbitrary distance. In addition, the optical properties of the fibers can influence the measurement or must be suppressed by suitable means, e.g., filters. This increases the equipment investment.
From WO 2006/081380 A2 is known a compact Raman spectrometer, which includes a laser light source, a low-resolution dispersion element, and a detection array, which are accommodated in a single housing. A sample to be examined is introduced into the compact spectrometer on an object holder. This compact spectrometer indeed does not require any wave guides extending outside the housing; on the other hand, it is not suitable for being connected to a process especially, an industrial process.